If at all possible, take the test after waking up from a good sleep. The test material should be the last thing you study before going to bed. If you drink caffeine (soda, or coffee, tea), then some of this can help before/during a test, but if you don't normally use caffeine, don't use it during a test. Fish oils are supposed to stimulate creativity in the mind. Omega 3, 6 & 9 fatty acids will provide these. Drink some water, this actually helps us think, but not to much that you'll have to be interrupted several times during the test.
Also think about what you will get after the test as a motivational thought.